Arianism

Arianism was a 4th-century heresy named after Arius (c.250-336), a priest in Alexandria, who taught that the Son of God was not God but rather a created being with a definite origin in time. In Arius's words, "there was [a time] when he was not."

Arius denied the full deity of the preexistent Son of God who became incarnate ("the Word (Jesus Christ) became flesh" John 1:14 - NKJV). He held that the Son, while divine and like God, was created by God as the agent through whom he created the universe, and thus that there was a time when the Son was not. The council at Nicaea in 325, led in its teachings by Athanasius, condemned Arianism and maintained that Christ was God from God, Light from Light, Very God from Very God, begotten not made (not created), and One in essence with the Father (homoousios/ομοούσιος - of the same essense rather than Arius' heretical homoiousios/ομοιούσιος - 'of a similar essence', producing the first version of the Nicene Creed.

Some forms of modern Protestantism appear to espouse a form of Arianism, referring to Jesus Christ as essentially distinct from God in terms that suggest that, as the Son, He is ontologically distinct from and inferior to the Father.

Source and further reading

St. Athanasius of Alexandria (page also has link to download 'Select Works and Letters' by St. Athanasius, including his treatise "On the Incarnation of the Word" and "Four Discourses Against the Arians" among others)